I just did a Windows Anytime Upgrade from Home to Ultimate. Which to my surprise was very quick and smooth. Anyways, I want to install Windows XP Mode to help with answering questions related to Windows XP on Super User and other Stack Exchange sites.

Before I download the 500MB file from Microsoft's site, I want to know if it prompts for a serial number? If it does, do I need to buy a Windows XP license? If doesn't require a new license but still require a serial number, do I use the Windows 7 Anytime Update from Home to Ultimate key?